After reading some posts on the Philipino Airsoft forum, I decided to install a relay to save the trigger from burning out.. At first I wanted to install a Mosfet, but after reading about burned out Mosfets from over heating, it seemed the 12v Relay was the more durable of the two alternatives, since I don't recall ever
having to replace a relay over the last 16 years of owning cars & motorcycles...

It took quite some doing to decipher the various diagrams, as I had a Denso relay which was different than all the ones I saw on various boards. Different meaning the prongs on this Relay were labeled 1,2,3,5; and others were labelled 30,86,87, etc... I finally figured out the proper wiring after looking at some manufacturer pictures & cross referencing them with the diagrams I saw for the other relays compared to
the Denso relay I had. (smallest relay I had, from 95 Honda Civic power window)

Edit: The relays shown on the other forum are this configuration:
http://oeiwcsnts1.omron.com/pdfcatal.nsf/0...D20G8HN0305.pdf

First off, the (4) prong relay would be used as a (3) prong, by connecting prongs 1 & 5.

The result is only (3) wires exiting from the rear of the box as opposed to (4).
*Great tip from ELAZ on this thread on the Philipino Airsoft forum:
http://www.filairsoft.com/modules.php?name...46612a7eab3cd14

Here is the diagram I came up with for my setup (based on the one ELAZ posted)
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_diagram.jpg

The Denso relay was removed from the housing and notches were dremelled out so it would fit inside the rear retractable stock insert on the MP5.
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_unit.JPG

The wiring in the gearbox with (3) wires exiting the back required some creative routing to make it all fit.
Trying to route (4) wires would be impossible, again thanks to the tip mentioned above.
First, the motor was mounted Negative pole towards the front, and Positive pole to the rear. This eliminated (1) wire from being routed across the Bevel gears path. You don't want to route a bunch of wires across there
due to the risk of getting chewed up by the motor pinion.
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_inbox.jpg

I used thinner gauge (equivelant to stock) for the switch wires which only provide juice to the relay's electro magnet (low voltage draw?) And thicker gauge wire for the motor to battery connections. The wire that goes
from the Neg battery directly the motor is thick, and the switch wire is merely spliced to it. Liquid electrical
tape was used at the joint where the wires join to prevent a possible short.
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_inclose.JPG

On a side note, because the gearbox shell conducts electricity, it may have been possible to create a
negative ground for the low voltage switch wires (like in cars & motorcycles) This would have eliminated
the second wire from being routed across the handle & bevel gear. I didnt bother trying it, but I think it
would work... It might cause problems with the selector plate however since it slides across the shell too.

The second thin switch (black) wire, had to be routed out the hole below the normal wires exit point. All (3)
wires are woven in such a way that they are only stacked (2) high where the gear box clamps down on them.

The drain? prong is connected directly to the motors positive pole with thicker (red) wire.
The final (2) prongs on the relay #1 & 5 are both connected to positive battery with thicker wire.

Deans mini plugs were used for the all connections except to the relay itself. There, modified plugs were used on the ground (shortened) down relay prongs (to allow room to move wires around)
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_plugin.jpg

I routed the fore grip battery extension wire up into the cocking tube space because the stock position
for those wires was annoying to line up. To do this, the groove for the cocking bolt spring guide was
dremelled deeper so the wires could be routed through the same hole.
http://mysite.verizon.net/res0cvsb/Airsoft/Relay_wire.jpg

At the end of the battery wire, I replaced the stock glass fuse capsule with a modern mini fuse holder.
This made more sense, because these fuses are cheaper, abundant, and I had a supply of them already
for my motorcycle.

Once reassembled, the relay sits flush in the little insert at the back of the gun. The wires plug in just behind
the relay and also serve to strap it down and hold it from moving out of the insert. The wires are cut to
length leaving just enough room to plug & unplug the connector.
http://mysite.verizon.net/res0cvsb/Airsoft...y_installed.jpg

Also shown in this pic, is a thin plastic insert I made to firm up the retractable stock arms. Works great, and
eliminiates almost all of the wobble.

I tested the connections with an ohm-meter (sp?) to ensure there were no shorts prior to pluging the
battery in.

I first tested the relay without the (red) drain wire to the motor plugged in. It clicked with each triggerpull
as I had hoped.. With the drain wire plugged in, the gun functioned properly in both semi and full auto.

So after many hours of planning & drawing diagrams at work, and even more time getting all the components & modifying them to fit in the MP5, the end result is:

Overall lower resistance wiring (thicker gauge, & deans plugs)
No more burnt trigger contacts!!
A better fuse holder
Easier take down of the gun (modified routing for battery wire)


A couple of pitfalls I encountered...

The space in the rear where the retractable stock locking lever is, was a tad too tight. To remedy this,
I sanded the disk down slightly (probably not enough to make a difference), I ground the notches off the
deans plugs on both sides, and (most importantly) counter sunk the hole where the disk tightens to the
locking lever so I could use a flush screw instead of the fat phillips screw.

The original plan was not to make the wires such a tight stretch, but rather leave enough slack to make
plugging & unplugging easier. This proved to be a pain, because the wires didnt want to get tucked inside
the body, and even after I did get them inside, fishing them out was a huge hassel..

The above mods to the retractable stock locking lever would not have had to be made if the connector
was tucked above the relay instead of connecting just behind it... I decided to make it work the other way,
because overall it does make taking the gun apart easier and the wires are less likely to get snagged
when re-assembling the gun because the wires are kept straight and in place..

Oh yeah, I also used the Birchwood Casey Aluminum Black on the gearbox outside surfaces.
Turned out so-so.... It was previously semi polished, but it dulled too quickly from all the grease.

Total cost was ~$10 for the connectors, shrink tubing, & another solder spool.
The relay was free, but only because I disconnected my cars horn previously as it got in the way of
a Ram-air I installed.. So I just swapped out the horn relay for the power window relay
(which was the smaller of the two)

This post has been edited by Noobie: Nov 29 2006, 11:53 PM